We have developed assays to detect oxidative lesions in specific genes and thus to quantitate their formation and repair. Oxidative DNA damage is generated by several different approaches including hydrogen peroxide, acridine orange, X-irradiation, irradiation with methylene blue, and treatment with 4NQO which forms at least one adduct with oxidative characteristics. The main lesion which we examine is 8-OH guanosine which can be detected by use of the FaPy glycosylase. This enzyme creates strand breaks in DNA at sites of the lesions, and the single stranded DNA can then be resolved on alkaline gels. We find that 8-OH guanosine is rapidly repaired in active genes in hamster and human cells. While it has been a general notion that there is no DNA repair in mitochondria, we now find that these organelles do have repair capacity. They can not, however, repair all lesions. They are capable of repairing DNA lesions created by monofunctional alkylating agents, but not UV induced pyrimidine dimers. We find fast repair of oxidative damage in mitochondrial DNA, and the mechanism is under investigation. One question is whether the repair in mitochondrial DNA is transcription coupled. We are investigating whether the common deletions in mitochondrial DNA seen in senescence and other conditions could be due to a localized deficiency in DNA repair.